1. Field of the Invention
This invention relates to a method for direct oxidation of a hydrocarbon and/or carbonaceous fluid fuel. More particularly, this invention relates to a method for generating electricity by directly oxidizing a hydrocarbon fluid fuel and/or a carbonaceous fluid fuel. This invention also relates to the use of electrochemical reactors including, but not limited to, solid oxide fuel cells (SOFC) for the direct oxidation of hydrocarbon and/or carbonaceous fluid fuels for generating electricity.
2. Description of Prior Art
Solid oxide fuel cells have grown in recognition as a viable high temperature fuel cell technology. There is no liquid electrolyte with its attending metal corrosion and electrolyte management problems. Rather, the electrolyte of the cells is made primarily from solid ceramic materials so as to survive the high temperature environment. The operating temperature of greater than about 600xc2x0 C. allows internal reforming, promotes rapid kinetics with non-precious materials, and produces high quality by-product heat for cogeneration or for use in a bottoming cycle. The high temperature of the solid oxide fuel cell, however, places stringent requirements on its materials. Because of the high operating temperatures of conventional solid oxide fuel cells (approximately 1000xc2x0 C.), the materials used in the cell components are limited by chemical stability in oxidizing and reducing environments, chemical stability of contacting materials, conductivity, and thermomechanical compatibility.
A solid oxide fuel cell generates electricity through the reduction of O2 to O2xe2x88x92 anions at the cathode, transfer of the anions through an electrolyte that is an electronic insulator (usually yttria-stabilized zirconia, YSZ), and finally the reduction of the O2xe2x88x92 anions by the fuel at the anode. Effectively, the chemical energy produced when the fuel is oxidized is given up to the electrons produced at the anode. Because the anode must be an electronic conductor and have a good thermal-expansion match with the electrolyte, it is usually composed of a ceramic-metal (cermet) composite, in which YSZ is the ceramic component. The most common anode materials for solid oxide fuel cells are nickel (Ni)-cermets prepared by high-temperature calcination of NiO and yttria-stabilized zirconia (YSZ) powders. High-temperature calcination is essential in order to obtain the necessary ionic conductivity in the YSZ. These Ni-cermets perform well for hydrogen (H2) fuels and allow internal steam reforming of hydrocarbons if there is sufficient water in the feed to the anode. Because Ni catalyzes the formation of graphite fibers in dry methane, it is necessary to operate anodes at steam/methane ratios greater than 3. However, there are significant advantages to be gained by operating under dry conditions. Progress in this area has been made using an entirely different type of anode, either based on ceria (See Eguchi, K, et al., Solid State Ionics, 52, 165 (1992); Mogensen, G., Journal of the Electrochemical Society, 141,2122 (1994); and Putna, E. S., et al., Langmuir, 11 4832 (1995) or perovskite anodes (See Baker, R. T., et al., Solid State Ionics, 72, 328 (1994); Asano, K., et al., Journal of the Electrochemical Society, 142, 3241 (1995); and Hiei, Y., et al., Solid State Ionics, 86-88, 1267 (1996).). These oxides do not, however, provide sufficient electronic conductivity. Replacement of Ni with other metals, including Co (See Sammes, N. M., et al., Journal of Materials Science, 31, 6060 (1996)), Fe (See Horite, T. et al., Journal of the Electrochemical Society, 143, 1161 (1996)) or Ag (Tsiplakides, D. et al., Journal of Catalysis, 185, 237 (1999)) has been considered; however, with the possible exception of Ag, these are likely to react with hydrocarbons in a way similar to that of Ni. Substitution of Ni with Cu would also be promising but for the fact that CuO melts at the calcination temperatures which are necessary for establishing the YSZ matrix in the anodes.
It is also well known that the addition of ceria to the anode improves performance. However, the high-temperature calcination utilized in conventional anode preparation causes ceria to react with YSZ, as a result of which performance is not enhanced to the extent that could be possible if formation of ceria-zirconia did not occur.
Fuel cells are attractive for power generation because much higher efficiencies can be achieved than in combustion engines. At present, most fuel cells require the use of H2 as the fuel, which is a severe limitation for their practical implementation as battery replacements and for transportation. However, the generation of electrical energy by the direct oxidation of hydrocarbons and carbonaceous materials in a fuel cell, without first reforming these fuels to H2, is a goal that many have viewed as unachievable.
Accordingly, it is one object of this invention to provide a method for generation of electricity by direct oxidation of hydrocarbons and other carbonaceous fuels.
It is another object of this invention to provide a solid oxide fuel cell for generating electricity using dry hydrocarbon and/or carbonaceous fluid fuels.
These and other objects of this invention are addressed by a method for generating electricity using a solid oxide fuel cell comprising an anode electrode, a cathode electrode and an electrolyte disposed between the anode electrode and the cathode electrode in which the anode electrode is contacted with at least one of a dry hydrocarbon fluid fuel (CxHy) and a dry carbonaceous fluid fuel (CxHyOz) the cathode is contacted with an oxidant, and the dry hydrocarbon fluid fuel and/or dry carbonaceous fluid fuel is directly oxidized by means of an electrochemical reaction, resulting in the generation of electricity. Suitable fluid fuels include, but are not limited to, methane, ethane, ethanol, propane, propanol and butane. By the term xe2x80x9cdryxe2x80x9d as used in connection with the hydrocarbon and carbonaceous fluid fuels utilized in the method of this invention, we mean fuels in which substantially no water in any form is present when undergoing oxidation in the fuel cell. To effect the direct oxidation of the hydrocarbon and/or carbonaceous fluid fuel, the anode electrode is constructed of a porous YSZ layer and a metal or metal alloy comprising an electron-conducting metal having an oxide form which melts at a temperature less than about 1550xc2x0 C. In accordance with one preferred embodiment, the electron-conducting metal is selected from the group consisting of Cu, Ni and alloys and mixtures thereof.
Anode electrodes for solid oxide fuel cells utilized in the method of this invention may be produced by a method in which a plurality of zircon fibers or other porous matrix material is mixed with a yttria-stabilized-zirconia (YSZ) powder, thereby forming a fiber/powder mixture. The fiber/powder mixture is then formed into a porous YSZ layer and calcined. Alternatively, a porous YSZ layer may be prepared by the tapecasting of YSZ with graphite pore formers. (See Corbin, S. F., Journal of American Ceramics Society, 82, 1693 (2000). The calcined porous YSZ layer is then impregnated with a metal-containing salt solution. As used herein, the term xe2x80x9cimpregnatedxe2x80x9d refers to a condition in which the metal-containing salt solution is disposed within the pores of the calcined porous YSZ layer. Contrary to conventional methods for solid oxide fuel cell anode electrode preparation, this method results in a YSZ layer which remains highly porous following high-temperature calcination to which any suitable metal, including Cu and Ni is then added by impregnation of the salt solution, after the high temperature calcination of the YSZ layer. In addition to enabling the use of metals whose oxides have a low melting temperature, this method of anode electrode production also allows catalytic materials, such as ceria and/or palladium (Pd) to be added in controlled amounts in a separate step.